Mustard is just the glue that holds the rub on. Ideally you like to see the rub get a wet look before you put the meat it in the smoker. So letting it sit in the fridge for a couple of hours wrapped in plastic wrap will do that. You can accomplish this with olive oil too. It just seems that mustard is the easiest way to keep the rub on, without changing the final taste. Maybe you should try a taste test. Use brown & yellow on one end of a rack of ribs & just yellow on the other. See if you can tell a difference.